Oh, finally, something that supports actual hierarchical state diagrams (that isn't Graphviz, no offense)... Mermaid's "You cannot define transitions between internal states belonging to different composite states" [1] has driven me up a wall for years.

  parentA.childA -> parentB.childB: voop
  parentB.childB -> parentA: vorp 

shouldn't be that hard!

[1] https://mermaid.ai/open-source/syntax/stateDiagram.html#comp...